Polymer blends or polyblends are mixtures of structurally different polymers or copolymers. Commercially important polyblends are generally mechanical polyblends which are made by melt-blending the various polymers in an extruder or other suitable intensive mixer. Polymer blending technology generally rests on the premise of property additivity, wherein the polymer blend has the combined properties which are found in each of the component polymers separately. A polymer blend can therefore have properties which are unavailable to the individual polymer components by themselves.
Blending of polymers is also cost efficient. Expensive polymers may be blended with inexpensive polymers to obtain a useful polyblend which has a cost:performance ratio that makes it very attractive for any given application. The standards of performance for any given application can therefore be met using blends of two known polymers without the need to develop completely new polymers and new plant equipment. There is a continuing need for novel and useful polymer blends.
Engineering plastics such as molding compositions should generally have a good balance of high tensile properties, stiffness, compressive and shear strength as well as impact resistance and should be easily moldable. For specific applications clear polymer blends may be desirable. Their physical properties should be reproducible and predictable and they should retain their physical properties over a wide range of heat and chemical conditions. Engineering plastics should be able to resist mechanical stress for long periods of time.
Blends of copolyester polymers with polycarbonates are known and have excellent melt processability and high impact strength. For example, European Pat. No. 01 11 810 discloses blends of an aromatic polycarbonate with a copolyester derived from 1,4-cyclohexanedimethanol, ethylene glycol and terephthalic and/or isophthalic acid monomer units. Blends of Bisphenol A polycarbonate containing 20-80 wt.% of the copolyester copolymer are hazy and exhibit two glass transition temperatures (Tg) which is indicative of immiscibility between the two polymers.
U.S. Pat. No. 4,237,325 and U.S. Pat. No. 4,246,381 disclose thermoplastic polymer blends containing a polyarylate derived from a dihydric phenol and an aromatic dicarboxylic acid, such as, for example bisphenol A polyesters and a copolyester derived from a cyclohexanedimethanol, an alkylene glycol and an aromatic dicarboxylic acid. These blends are described as having an excellent melt processability, high impact strength and weatherability.
A need continues to exist, however, for new and miscible polymer blends with improved combinations of properties.